1. Field of the Invention
This invention relates to a method of plugging of the more permeable strata of subterranean reservoirs having non-uniform permeability. More particularly, the invention relates to such a method to provide better control of fluids injected into a reservoir during enhanced oil recovery operations or withdrawn from a reservoir during production operations.
2. Description of the Prior Art
When fluids flow through reservoirs having sections of varying permeability, a higher percentage of the fluids tends to flow through those sections having a higher permeability. It is often desired to decrease or stop the flow of fluids through these sections of higher permeability. For example, in the enhanced recovery of petroleum by flooding, a displacing fluid is injected into the reservoir via an injection well to displace the petroleum through the reservoir toward a producing well.
In the normal flooding operation, maximum oil recovery is obtained when the driven fluid builds up in a wide band in front of the driving fluid which moves uniformly towards the producing well. To keep this bank of oil intact, and constantly moving towards the producing well, a substantially uniform permeability must exist throughout the reservoir. If this uniform permeability does not exist, or is not provided, the flooding fluid will seek the areas of high permeability, and channeling occurs with the consequent loss of some driving fluid energy and the appearance of excessive amounts of driving fluid in the producing well.
There is a tendency for the injected fluid to follow the path of least resistance, pass mostly through the portions of the reservoir having the highest permeability and bypass the petroleum present in the less permeable portions of the reservoir. If these high permeability zones of the reservoir were plugged, the injected fluid would be forced to flow into the less permeable portions of the reservoir and displace a higher percentage of the petroleum present in the entire reservoir. Similarly, in the production of oil, producing wells sometimes produce water and/or gas along with oil. The water and gas often are produced through the portions of the reservoir having a relatively high permeability. If the zones through which water and gas are produced could be at least partially plugged, a higher percentage of the produced fluids would be the desired oil phase.
A wide variety of materials have been proposed for use in plugging subterranean reservoirs. Plugging by injection of a solution of a polymer solution and a solution of a material, such as a multivalent metal cation, which reacts with the polymer to form a gel, agglomerate, precipitate or other plug is shown in U.S. Pat. Nos. 3,396,790 to Eaton, 3,799,262 to Knight, 3,581,824 to Hurd, 3,762,476 to Gall, 3,909,423 to Hessert et al., 3,795,276 to Eilers et al., 3,658,129 to Lanning et al. and 3,809,160 to Routson. Injection of a solution of an alkali metal silicate and a solution of a material which reacts with the silicate to form a gel or a plug is well-known as shown in many patents, such as U.S. Pat. Nos. 3,530,937 to Bernard, U.S. Pat. No. 3,386,509 to Froning and 3,402,588 to Andresen. U.S. Pat. No. 3,741,307 to Sandiford et al. describes injection of a thickened aqueous polymer solution followed by a solution of a liquid agent which reacts in the reservoir to form a plugging material. An example of suitable reactants include sodium silicate and a delayed gelling agent therefor, such as ammonium sulfate. Another approach has been to produce a double plug. Thus, U.S. Pat. No. 3,306,870 to Eilers et al. discloses injecting an aqueous solution of an acrylamide polymer and a material which reacts with the polymer to form a first plug. The composition may be preceded or followed by a slurry of an expansive hydraulic cement which reacts in the reservoir to form a second plug.
U.S. patent application Ser. No. 667,694 filed Mar. 17, 1976 by Burton B. Sandiford, now U.S. Pat. No. 4,009,755, describes the formation of a combination plug by injecting into a reservoir a polymer and a material that reacts with the polymer to form a polymercontaining plug and an alkali metal silicate and a material which reacts with the silicate to form a separate silicate-containing plug.
In spite of the wide variety of plug-forming compositions and methods previously suggested, need remains for even more effective plugging materials, especially for use in reservoirs having channels or zones of high permeability which are especially difficult to seal off. The flow rate of enhanced oil recovery fluids through these high permeability channels is often as high as 0.1 mile per hour and in some instances may be as high as 0.5 mile per hour or higher. At these high flow rates, it is difficult to design a plugging method wherein the plugging material remains fluid and pumpable long enough to enable it to be properly positioned in the channel, yet sets up within a relatively short time to form a firm plug. This is especially true when the channel exists between wells which are a relatively short distance apart, for example 500 feet.
Accordingly, a principal object of this invention is to provide a method for controlling the permeability of a subterranean reservoir of nonuniform permeability.
Another object of the invention is to provide a method for reducing channeling of a flooding medium from an injection well to a producing well via a zone of high permeability.
Still another object is to provide a method for selectively plugging water-producing zones in a subterranean reservoir.
A further object is to plug especially onerous and difficult to plug high permeability channels in a subterranean reservoir.
A still further object is to provide a mixture of two plugging materials for plugging high permeability channels in a subterranean reservoir.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.